European patent publication 0,196,856 discloses a laser for detecting a gas which operates at two emission wavelengths and is modulated by sinusoidal oscillations of a piezoelectrically-driven resonator mirror. The laser here is an IR--He--Ne laser having a methane cell. The modulation takes place at 1 kHz in such a manner that the sum of the intensities at both wavelengths together remains substantially constant. Nothing further is provided as to detection in this publication.
The detection of ammonia with a two-wavelength CO.sub.2 laser is described in a paper of A. G. Gerasimchuk et al entitled "Frequency-Switching Waveguide CO.sub.2 Laser for Atmospheric Pollution Monitoring" published in Sov. J. Quantum Electron 12 (1), January 1982, pages 110 to 112. The wavelength modulation takes place utilizing square wave control pulses via a piezoelectrically-driven mirror. A synchronous detector is provided.
The detection of NH.sub.3 in flue gas from an industrial furnace utilizing two wavelengths of isotopically-selected .sup.13 CO.sub.2 lasers is described in a paper of A. Stein et al entitled "Carbon Dioxide Laser Monitor for NH.sub.3 in Flue Gas", Applied Optics, Volume 22, No. 21 (November 1983), pages 3378 to 3381 (see also U.S. Pat. Nos. 4,471,220 and 5,002,391). The paper of A. Stein et al and U.S. Pat. No. 4,471,220 disclose the use of two separate lasers for the two wavelengths and two mechanical choppers. U.S. Pat. No. 5,002,391 discloses a single laser having a mirror or grating which is shifted piezoelectrically with square wave pulses in order to alternately generate one of the two wavelengths. A synchronized mechanical chopper effects a blanking after each pulse pair at both wavelengths. Pyroelectric detectors and sample-and-hold circuits are utilized for detection.
British patent publication 2,127,537 discloses a gas detector having a laser which is preferably a CO.sub.2 laser without a grating. The laser can emit by means of a piezoelectrically displaced resonator mirror at different wavelengths. The following are mounted in the measurement beam path: a mechanical chopper, a cooled photodetector and a phase-sensitive detector circuit. A reference cell having a pyroelectric detector is required.